For a moment the two trains run together, side by side. In that frozen moment, Elspeth witnesses a murder. Helplessly, she stares out of her carriage window as a man remorselessly tightens his grip around a woman’s throat. The body crumples, and the other train draws away.

But who, apart from Miss Marple, would take her story seriously? After all, there were no suspects, no other witnesses, and no corpse.